This invention relates generally to indicator devices such as differential pressure gauges and the like, and more particularly to devices of this type which incorporate a driving permanent magnet that moves in response to changes in pressure conditions, and a follower permanent magnet which reacts with the driving magnet and which carries an indicating member intended to move across a scale.
In particular, the present invention involves improvements in the construction disclosed and claimed in may U.S. Pat. No. 3,429,291 dated Feb. 25, 1969 and entitled DIFFERENTIAL-PRESSURE RESPONSIVE INDICATOR. The device shown in my above patent is of the type intended to measure differential pressures for gas, liquids such as water, or other chemical substances, etc., that is, differences in pressure for instance between two oil lines that are intended to be connected to oppositely disposed ports of the indicator. As shown in FIG. 1 of the patent, the indicator has a scale 38 and a pointer 36 carried by a follower magnet 26; a driving magnet 12 is disposed against and movable with a piston 16 disposed in a cylinder 14. A spring 22 can move the magnet 12 and piston 16 toward the left in the absence of a pressure difference between the left and right ports.
While the above construction has found important use for many years and has provided a very satisfactory performance over this period of time, several areas of the design were susceptible to improvement. With the particular instrument shown in the patent, the tolerances on the spring 22 were such that there could occur slight variations in the force exerted by the spring on the magnet 12 from unit to unit, these being on the order of .+-.10% for commercial springs. Under such circumstances, it was necessary to secure the pointer 36 to the magnet 26 by hand at a suitable zero point setting; however slight variations in the spring constant could give rise to undesired expansion or contraction of the scale over which the pointer 36 moved, depending on the particular stiffness of the spring. The characteristic variation between springs was compensated for by calibrating the scale plate 38 for each individual instrument and thereafter spotting a series of points thereon, marking such points with scale lines and applying numerals or other markings to the scale plate 38 by hand drawing, or else by transfer of decals or decal-like letters. While such a procedure provided high accuracy, since each scale plate 38 was calibrated individually the arrangement proved to be costly from the manufacturing standpoint due to the time involved in such individual calibration.
Attempts to improve the calibrating procedure for the instrument so as to eliminate the necessity for unit-by-unit attention and work did not meet with much success, because of the relatively poor tolerances obtainable with the springs 22. Accordingly, it was considered that easy calibration of these instruments, whereby pre-printed dials could be used, was not practical by economical mechanical means.
In addition to the relatively high manufacturing cost involved with individual calibration of each instrument, the patented construction did not incorporate any arrangement for recalibrating the unit in the field, or else modifying the instrument to any extent in order to adapt it to slightly different applications. Accordingly, there arose problems regarding the distributor's inventory, as to just what instruments should be kept in stock.
It can be readily appreciated that differential pressure gauges of the type noted above are generally used in a number of different applications, and it was considered important to be able to adapt a particular instrument to suit applications requiring minor deviations from the basic instrument design. The device shown in the above patent did not have such a capability.
Other prior differential pressure gauges also had the same drawbacks, as regards tolerances of commercial compression springs; and the problem of compensating for the wide variations in spring stiffness, spring length, wire twist and other variables appears to be widespread throughout industry. In many instances, instrument grade springs or mechanical adjustment means were resorted to, which added considerable cost to magnetic drive instruments wherein, for example, one permanent magnet was used as a driving magnet and another permanent magnet was used as a follower. In this type of instrument, the reactive forces are normally not extremely strong, and therefore the friction factor had to be considered, as well, thereby introducing still another variable where the desire was to eliminate variables and simplify the structure, rather than adding components and making the structure more complicated.